In transmitters for wireless communications systems, the power amplifiers (PAs) often have to simultaneously amplify many radio channels (frequencies), which are spread across a fairly wide bandwidth. They also have to do this efficiently, in order to reduce power consumption and need for cooling. High linearity is also required, since nonlinear amplifiers would cause leakage of interfering signal energy to other channels in the system.
The amplitude probability density of a mix of sufficiently many independent radio frequency (RF) channels, or of a multi-user Code Division Multiple Access signal (CDMA signal), tends to be close to a Rayleigh distribution having a large peak-to-average power ratio. Since a conventional RF PA generally has an efficiency proportional to its output amplitude, its average efficiency is very low for such signals.
In response to the low efficiency of conventional linear PAs, many methods have been proposed. Two of the most promising are the Chireix outphasing method [1] and the Doherty method [2], which are based on composite power amplifiers including more than one amplifier.
Base stations for wireless communications systems are often equipped with several transmitters (and therefor several power amplifiers). Reasons for this can be that the base station is serving several geographical areas (‘sectors’ or ‘cells’) or that an array of several antenna elements is used. In the first case the number of independent transmitters is typically three. Combinations of the two methods are also possible.
In the case of a three-sector base station, the three antennas usually output totally independent signals (three different mixtures of signals to users in specific sectors) amplified by three separate power amplifiers.
In the case of an antenna array, different users within a sector are at different locations, and therefore the signals to the individual antennas in the array (different mixtures of phase shifted versions of signals to users) are generally partly correlated. The correlation is generally higher for lower angular spread of users (narrower sectors) and smaller spacing between the antennas. Also in this case separate power amplifiers are used for each antenna in the array.
As noted above state of the art power amplifiers (Chireix and Doherty type amplifiers) achieve higher efficiency than class B amplifiers by having two or more independently controlled constituent amplifiers (transistors) per composite amplifier. A problem with this approach is the large number of control paths (signal generation, up-conversion, amplification, etc) when the amplifiers are used in multiple input multiple output applications, such as multisector base stations or adaptive antenna arrays. This is due to the fact that each composite amplifier requires at least twice the number of control paths as compared to compared to a non-composite amplifier.